The Skeletal System - Human Biology PDF

Human Biology 3. The Skeletal System Professor Ebbert, Spring 2025 Lecture objectives Describe how osteocytes, osteoblasts, and osteoclasts are involved in bone remodeling, being specific about the role of the plasma memb...

Human Biology 3. The Skeletal System Professor Ebbert, Spring 2025 Lecture objectives Describe how osteocytes, osteoblasts, and osteoclasts are involved in bone remodeling, being specific about the role of the plasma membrane in each cell type. Describe the location and state the function of adipose tissue, fibrocartilage, DICT, DRCT and hyaline cartilage in synovial joints. Explain how the molecules, tissues, and other structures that make up the skeletal system allow it to resist compression, tension, and friction. This material is covered in Chapter 6 (bone) and Chapter 9 (joints). You can also find information about cartilage, bone, and gap junctions in Chapter 4. Notes I. What is the skeletal system? A. The skeletal system includes bones, joints, and the connective tissue that holds it together. B. Functions of the skeletal system include: 1. Support: provide a framework for body, e.g., legs support trunk. 2. Protect from compression, tension, and friction: e.g., skull protects brain from blows to the head. 3. Allow movement: gives skeletal muscles a place to attach, forms joints so the skeleton can bend. 4. Store fat, calcium ions, and phosphate ions. 5. Make blood in bone marrow. C. Major problem for skeletal system 1. Has to allow movement. Movement causes friction and reduces stability. 2. Has to provide support and protection, so it can’t fall apart under compression, tension, and friction. 3. The best way to resist compression, tension and friction is to not move. Example of how there are no perfect designs for organisms: the structures needed for one function often interfere with other functions. A very stable synovial joint won’t move, for instance, and a very mobile synovial joint isn’t stable. II. What is cartilage? A. Very thin CT made by chondroblasts and maintained by chondrocytes. Extracellular matrix includes a firm ground substance (which resists compression) and many collagen 1 and collagen 2 fibers (which resist tension). B. Types of cartilage 1. Hyaline cartilage: mainly collagen 2 fibers. Has more ground substance than the other types of cartilage, and it contains a lot of water. That makes it especially resistant to compression. Example locations: costal cartilage, articular cartilage. 2. Fibrocartilage: mainly collagen 1 fibers packed in layers. These thick layers resist compression. Example location: the menisci of the knee. 3. Elastic cartilage: elastic fibers in a web and collagen 2 fibers. Example location: epiglottis. ©MAEbbert, 2025 BIO 115 1 C. Perichondrium covers most cartilage. 1. Made of DICT. Resists tension. Resists compression when cartilage is compressed. 2. Blood vessels and nerves run through perichondrium, but not into cartilage. 3. Ligaments and tendons fuse with perichondrium. D. Integration: avascular and has no nerves. III. What is bone? A. Made by osteoblasts, broken down by osteoclasts, and maintained by osteocytes. 1. Bone cells live in tiny spaces in the extracellular matrix. 2. These spaces are connected to each other by tiny canals. Bone cells extend their membrane into the canals and form gap junctions with other cells. a. Gap junctions are membrane attachments. They include with hollow tubes that open into the cytosol of both cells. b. Allow ions and molecules to travel directly and quickly from the inside of one cell to the inside of another. This rapid movement lead to rapid communication between cells. B. Extracellular matrix 1. Many collagen 1 fibers. 2. Ground substance is hard (resists compression) because mineral (e.g., calcium and phosphate) crystals get added to it. C. Types of bone 1. Compact bone is found on the surface of the bone. a. Compact bone is made of osteons that are shaped like poles. These resist compression. b. Osteons are layers of tissue, one inside the next. Each layer has collagen fibers running at a different angle to the next, and so they resist tension in any direction. c. Central canal connects to the tiny canals. 2. Spongy bone is found in interior of bone. a. Bone is arranged in webs (“trabeculae”) with bone marrow in the spaces. b. Compressive force is distributed throughout the web, so spongy bone resists compression. D. Periosteum covers most of the bone. 1. Made of DICT. Resists tension. 2. Blood vessels and nerves run through periosteum and into the bone. 3. Ligaments and tendons fuse with periosteum. E. Integration 1. Cardiovascular system: bone is richly vascularized. a. Blood is made in bone marrow. Cells in spongy bone get nutrients and oxygen from the blood in the marrow. b. Cells in compact bone are supplied by the blood vessels running through the central canals. 2. Nervous system a. Nerves run through central canal of osteons. b. Sensory receptors in joints and bones collect information about pain, pressure, body movement, and body position. ©MAEbbert, 2025 BIO 115 2 IV. How is bone remodeled? A. Bone remodeling is adding or subtracting bone tissue for reasons other than growth or repair. B. Response to mechanical stress 1. Osteocytes sense whether and where bone is under pressure, from, for example, muscles pulling on it or another bone rubbing on it. 2. Osteocytes tell osteoblasts to become more active at the specific locations where bone is stressed. They secrete more fibers and ground substance, which then gets calcified. 3. Osteocytes tell osteoclasts to become more active where bone is less stressed. a. Osteoclasts secrete HCl (breaks up crystals) and enzymes (break down collagen fibers and ground substance). b. Osteoclast plasma membranes are wavy, which increases the membrane’s surface area. C. Remodeling also happens in response to hormones. If calcium blood levels are low, hormones that increase osteoclast activity are released. As calcium levels rise, hormone release declines, and, because they are no longer stimulated, osteoclasts are less active than osteoblasts. As a result, calcium gets stored in the bone. V. What are joints? A. Joints attach bones to other bones, cartilage, or teeth. B. Joints with one function: making the skeleton stable. Allow little if any movement. a. Fibrous joints. Joined by collagen fibers. E.g., sutures, which eventually lose their fibers and fuse shut, and gomphoses, where teeth are attached to bone. b. Cartilaginous joints. Joined by cartilage. E.g., symphyses between hip bones. C. Synovial joints have two functions: stabilize the skeleton and allow it to move. VI. What is in a synovial joint? A. Articular capsule has two layers 1. Fibrous capsule (superficial layer). DICT. Grows into periosteum. Holds the bone and joint together tightly. 2. Synovial membrane (deep layer). Specialized CT with lots of blood vessels. Uses blood to make synovial fluid, which is slippery (resist friction) and has lots of nutrients and oxygen. B. Joint cavity. The space between the bones, filled with synovial fluid and lined with synovial membrane. C. Articular cartilage. Hyaline cartilage that covers parts of the bones that meet in the joint. No perichondrium so this cartilage gets supplied by synovial fluid. The surface is slippery and doesn’t compress. D. Ligaments: bands of DRCT that connect bone to bone across a joint. They improve stability by resisting tension. ©MAEbbert, 2025 BIO 115 3 E. Bursae: flexible sacs of synovial membrane filled with synovial fluid. They are tucked between two structures, like a ligament and a bone, that slide past each other, resisting friction. F. Fibrocartilage pads and fat pads. 1. Both improve how the bones fit together, which helps keep the joint stable. 2. Both also resist compression, so they provide a cushion. 3. Examples of fibrocartilage pads include the two knee menisci. VII. How does the skeletal system solve its problems? A. Problem: compression. Solutions include osteons, mineral crystals, trabeculae, fat pads, fibrocartilage pads, synovial fluid, perichondrium, articular cartilage. B. Problem: tension. Solutions include osteons, periosteum, fibrous capsule, ligaments, perichondrium. C. Problem: friction in synovial joints caused by bones, tendons, and ligaments rubbing against each other. Solutions include articular cartilage, synovial fluid, bursae. ©MAEbbert, 2025 BIO 115 4